Among the fleet of current interplanetary spacecraft, I think Dawn is perhaps the most interesting. It will be the first spacecraft to enter orbit around a planet(oid) and then fly to a second one to enter orbit there. I just can't wait to see how it turns out.
Let's not forget Deep Impact. Flew past two asteroids, and even threw something into the first one while taking pictures (with an incredible autonavigation system). Pretty awesome if you ask me.
That was pretty novel, but I still think Dawn is one of the most interesting still because of the ion thruster and re-orbit of ceres. The neat thing about DI (now called Epoxi and run by UMD, btw) is that it's now become a sort of flying testbed for all sorts of experimental flight and communication software. See "DINET"
Dawn is also quite interesting because it will be studying some of the largest objects in the Solar System that have never been visited by spacecraft before. We've studied the moons of all of the planets in detail, even the tiny ones. Yet Ceres and Vesta are entire worlds, with internal differentiation into a core, mantel, and crust, and complex surface features such as faults and perhaps even volcanoes. I suspect that there are many exciting surprises waiting for us when we finally get a good look at these dwarf planets.
Pretty exciting considering how far we've advanced. In the 1960s (an era when humans walked on the moon, even) we could do little better than send probes on flybys of planets and return a handful of low res photos and other data. The Mariner 4 mission to Mars returned less than 700 kilobytes worth of data, and left us with the impression that Mars was very much more like our Moon than anything else, it took many later missions for our more modern understanding of Mars to firm up. In contrast, Dawn uses ion engines to rendezvous with not one but two asteroids, and will study them with an impressive suite of instruments including an incredibly advanced imaging spectrometer. Each image that Dawn takes will return orders of magnitude more useful scientific data than the entire Mariner 4 mission. Meanwhile, there are other missions such as the Mars Science Laboratory, WMAP and Planck, JWST, JUNO, which are equally as astounding as measures of how far we've advanced in just a few decades.